1. Field of the Invention
The present invention relates to a general mobile radio communication system and, in particular, to an OFDM radio communication system supporting backhaul communication of eNBs. The eNBs of radio communication system are capable of performing coordinated transmission through backhaul communication. This is referred to as Coordinated Multipoint processing (CoMP).
Throughout the present invention, 3GPP LTE Release 8 system is referred to a legacy system, in-development Release 10 system is the system to which the embodiments of the present invention are applied. Of course, the present invention can be applied to other mobile communication systems such as IEEE 802.16 (WiMax) system.
2. Description of the Related Art
CoMP Transmission
Basically, CoMP processing operations can be categorized into following categories:
Joint processing (JP): In JP, eNBs perform scheduling jointly and may not open.                Transmission point(s) for CoMP transmission/reception operation: point or as set of points transmitting physical downlink shared channel (PDSCH) to UE dynamically        
(1) Transmission point(s) are a subset of the coordination set for CoMP transmission.
(2) In JP, the transmission points for CoMP transmission are the points of a coordination set for CoMP processing.
(3) In dynamic cell selection, transmission is available at point at a time, and the transmission point can be changed dynamically in every subframe.
(4) In coordinated scheduling/beamforming, the transmission point for CoMP transmission is “serving cell.”
2. Reporting Set for CoMP Processing
Set of the cells in which channel status/statistic information related to the link to the UE
(1) The reporting set can be identical with the cooperation set
(2) The real UE report can select downlinks of the cell for transmitting real feedback information.
Essential Feedback Between UE and eNB for CoMP Transmission/Reception Operation
In order to support CoMP transmission/reception operation, it is necessary to receive UE feedback for multiple communication channels from multiple eNBs. In the legacy system, the multipoint transmission is possible only with multiple antennas of the same eNB, and thus the following information is necessary for extending the multipoint transmission to the case of multiple eNBs.
1. In order to support JP, entire/partial channel state information of each channel is required, and such information can include channel state information (CSI), plural channel quality indicators (CQIs) of each channel, and optimum precoding matrix indicator (PMI).
2. In order to support coordinated scheduling/beamforming, plural CQIs of sub-bands predetermined per channel or worst PMIs of neighbor cells that are influenced as interference are required.
The most of feedback formats for supporting CoMP transmission/reception operation cause significant overhead. It is an object of the present invention to reduce the overhead of feedback for CoMP transmission/reception operation.
Complex Delay Profile in CoMP Transmission
In OFDM-based system, Cyclic Prefix (CP) is added at the head of each OFDM symbol. The CP is added to each OFDM symbol to absorb Inter-Symbol Interference by the previous symbols caused by the paths delayed on the communication channel. Typically, multipath fading channel is expressed as delay profile of gain to the delay time of the delayed paths. The OFDM system is designed such that CP absorbs the most of the inter-symbol interference. Also, the OFDM system can adjust the length of CP in match with the channels having different maximum delays.
Meanwhile, in case of CoMP transmission in a single UE communicates with multiple eNBs, the communication channel corresponds to a complex channel composed of a plurality of multipath channels. For example, in case of downlink (DL) as shown in FIG. 3, the UE is synchronized with the serving eNB but the signal from the cooperated eNB1 arrives with extra delay ΔD1 according to the extra electric wave delay. Such two channels are aggregated by the receiver of the UE and, as a consequence, the maximum delay becomes max(D0, ΔD1+D1) and can be longer than the predetermined CP according to the geographical positions of the eNBs and corresponding channel states.
If the maximum delay of the complex channel is longer than the predetermined CP, the system performance is degraded by intra-symbol interference and thus it may not be preferably to apply CoMP transmission. Accordingly, when selecting a partner for CoMP transmission, it is important to determine whether the neighbor eNB is appropriate for the CoMP transmission for a specific UE.
However, there is no mechanism, specified in the present mobile communication system standards, for a UE to report the timing information of the signal received in the neighbor cell yet.